1. Field of the Invention
The present invention relates to an engine torque control apparatus in an automatic transmission provided with a torque converter.
2. Description of the Prior Art
Conventionally, a V-belt type continuously variable transmission (hereinafter referred to as “belt CVT”) has been used as an automatic transmission for a vehicle. The belt CVT is constructed such that a primary pulley and a secondary pulley whose groove widths are variably controlled according to oil pressure supports a V belt by sandwiching it, and power is transmitted by contact frictional force of the V belt.
An example of the belt CVT has been disclosed in Japanese Laid-Open Patent Publication No. 11-37237. In the belt CVT disclosed in this publication, the thrust of pulleys is determined based on the input torque and the gear ratio, and it is converted to oil pressure value according to predetermined values indicative of the pressure receiving areas of the secondary pulley and the primary pulley, or the like, and the oil pressure value is supplied as target line pressure to a transmission mechanism.
Further, this belt CVT is provided with a torque converter that is disposed between an engine and the transmission mechanism, so that the torque inputted from the engine is subjected to amplification or the like and then transmitted to the transmission mechanism.
In the above described conventional belt CVT provided with the torque converter, the torque converter amplifies output torque from the engine, and transmits the torque to the transmission mechanism. The output torque from the engine is reduced by torque-down control so as to prevent an excessive torque greater than the allowable input torque of the belt CVT from being inputted to the transmission mechanism of the belt CVT due to torque amplification. The amount of torque-down by such torque-down control is found from the throttle valve opening and the engine speed, and the torque-down amount is set to become smaller as the engine speed increases, and to become greater as the throttle valve opening increases.
However, torque discharge occurs due to rotational inertia of the engine and the belt CVT, and hence the torque-down amount found from the throttle valve opening and the engine speed must include a margin for absorbing the torque discharged due to the rotational inertia. Since the margin for absorbing the discharged torque is required, the torque-down amount must be set to a great value, which deteriorates the engine performance in the case when the vehicle starts running with a stall condition, the case when the vehicle climbs a critical up-slope, and in other cases.